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RE: Expansive Soil

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The 5 microns is a "clay" fraction.  Soil particle this size and smaller are considered clay.  I think the UBC is trying for 10% clay.
Although you may have some volume change potential with a PI of 15 it may not be significant enough to actually be a problem.
When in doubt run a swell test.  There are several variants, I like to actually swell the soil and then re-load to the initial void ratio to get the actually uplift pressure. 
Anyway.  PI is only a guestimate method to determine swelling soils.  A PI of 15 in my area would have little actual swell potential if at a reasonable moisture.  YMMV.
 -----Original Message-----
From: GSKWY(--nospam--at) [mailto:GSKWY(--nospam--at)]
Sent: Saturday, April 09, 2005 7:40 AM
To: seaint(--nospam--at)
Subject: Expansive Soil

Per IBC Section 1802.3.2.
Soils are considered expansive if:
· They have a Plasticity Index (PI) of 15 or greater, determined in accordance with ASTM D 4318.
· More than 10 percent of the soil particles pass a No. 200 (75 µ) sieve, determined in accordance with ASTM D 422.
· More than 10 percent of the soil particles are less than 5 micrometers in size, determined in accordance with ASTM D 422.
These last two requirements don't seem to make sense.  If more than 10% of the particles are smaller than 5 micrometers (5 microns),  obviously more than 10% are smaller than the No. 200 sieve (75 microns).
It is different tests - the test for the 5 micron size is done with a hydrometer,  but it still doesn't seem to make sense.
Is there more to this?  I would assume it came from the UBC,  since a lot of the soils stuff in the IBC seems to have come from the UBC.
Gail Kelley